This invention relates to a hydraulic lubricating oil composition and method for providing improved thermal stability properties. More particularly this invention is directed to a hydraulic lubricating oil composition of relatively high viscosity index (VI) with good antiwear, anticorrosion and thermal stability properties comprising a major amount of paraffinic mineral oil and a particular combination of a basic zinc dialkyl dithiophosphate and 2,6 di-tertiary butyl phenol.
The field of lubricants and lubricating oils has been extensively developed over the years. Because of the wide variety of applications and conditions a large number of different oil compositions with a plurality of additives have been developed and manufactured. However, because of the complexity of the properties associated with such lubricants and the relationship of the different components to one another, it is oftentimes difficult to develop suitable lubricant compositions for a particular application.
The use of metal dithiophosphates as antiwear additives and also as antioxidants in lubricating oils has long been known. Various antioxidants including phenolic compounds and particularly hindered phenols are also wellknown additives for lubricating oils as disclosed in "Lubricant Additives" by C. V. Smalheer and R. Kennedy Smith 1967, pp. 6-11; Kirk-Othmer "Encyclopedia of Chemical Technology," Second Edition, Vol. 12, 1967, pp. 574-575 and U.S. Pat. Nos. 2,202,877; 2,265,582; 3,032,502 and 3,929,654.
While the use of various compounds as antioxidants and antiwear additives in lubricating oils is known as previously indicated, nevertheless, it was difficult to develop a hydraulic oil composition having a paraffinic mineral oil basestock with high VI and with the requisite antiwear, anticorrosion and thermal stability properties.